Flip chip technology has grown rapidly in recent years, since it allows a decreased chip footprint while simultaneously increasing the number of possible input/output contacts. This is because flip chip technology takes advantage of the entire chip area for input/output contacts, instead of just the chip periphery, as with wire-bonded integrated circuit chips. Various techniques exist for solder bumping wafers, such as evaporation, plating, solder paste screening, and more recently, injection-molded solder (IMS).
Typically, bumped wafers are diced in a next step into separate chips. For direct chip attach (DCA), silicon chips are bonded directly to a laminate substrate. DCA is a rapidly growing packaging technology since it requires a smallest amount of area on the laminate, has the highest height, and is lighter than other packages. Additionally, flip chip applications provide better electrical characteristics and better cooling than wire-bond package chips. However, since there is a significant coefficient of thermal expansion (CTE) mismatch between the integrated circuit chip, typically silicon-based, and the laminate material, such as an organic material, DCA bonded chips must conventionally be underfilled with an adhesive support material. Advantageously, this underfilling greatly increases the fatigue life of the solder bump connections between the integrated circuit chip and the laminate substrate.
One difficulty with this approach, however, is the lack of reworkability of the integrated circuit chip once adhesively bonded to the laminate substrate. Reworkability, particularly in multichip packaging, is a significant issue in making such packaging financially profitable. The underfill adhesive needed to address the coefficient of thermal expansion mismatch between a silicon-based integrated circuit chip and the supporting laminate substrate, prevents (or at least significantly inhibits) the ability to rework the integrated circuit chip assembly. Because of the underfill adhesive, such packaging does not lend itself readily to removal of the integrated circuit chip or to site cleaning of the individual contacts interconnecting the integrated circuit chip and substrate. This remains true notwithstanding the numerous attempts in the art to develop a reworkable underfill adhesive, principally due to the conflicting requirements and the different roles the underfill adhesive needs to perform imposes on the constituent materials.